This invention relates to liquid crystal device alignment.
Liquid crystal devices typically comprise a thin layer of a liquid crystal material contained between cell walls or substrates. Optically transparent electrode structures on the walls allow an electric field to be applied across the layer, causing a re-ordering of the liquid crystal molecules.
Many different modes of liquid crystal devices are known in the art, for example the twisted nematic device the cholesteric phase change device, the dynamic scattering device, the supertwisted nematic device and the surface stabilised ferroelectric device modes. It is well known in all of these device modes to provide a surface on the interior walls of the device which will control the alignment of the liquid crystal fluid in close proximity to the surface. For many applications of liquid crystal devices, such a treatment is considered necessary in order to impose a particular configuration on the alignment of the liquid crystal fluid throughout the device and/or to provide an optical appearance in the device which is free of apparent optical defect. The particular significance of this factor for different classes of liquid crystal device is described in greater detail below.
The terms azimuth or azimuthal is used hen in to define the molecular (or director n) alignment angle movement or energy in the plane of the substrate surface. The terms zenith or zenithal is used herein to define the molecular alignment angle movement or energy in a plane normal to the substrate surface.
In respect of use of nematic and long pitch cholesteric materials for devices known as twisted nematic liquid crystal devices, the relevance of alignment and the problems associated therewith are as follows.
In order to provide displays with a large number of addressable elements it is common to make the electrodes as a series of row electrodes on one wall and a series of column electrodes on the other cell wall. These form typically an x,y matrix of addressable elements or pixels and for twisted nematic types of device are commonly addressed using rms addressing methods.
Twisted nematic (TN) and phase change devices are switched to an ON state by application of a suitable voltage and allowed to switch to an OFF state when the applied voltage falls below a lower voltage level, i.e. these devices are monostable. For a twisted nematic type of device (90xc2x0 or 270xc2x0 twist as in U.S. Pat. No. 4,596,446) the number of elements that can be rms addressed is limited by the steepness of a device transmission verses voltage curve (as described by Alt and Pleschko in IEEE Trans ED vol ED 21, (1974) P.146-155). One way of improving the number of pixels is to incorporate thin film transistors adjacent to each pixel; such displays are termed active matrix displays.
An advantage of nematic types of devices is the relatively low voltage requirements. They are also mechanically stable and have a wide temperature operating range. This allows for the construction of small and portable battery powered displays. An alternative twisted nematic device is one which is switched from a non-twisted state at zero volts to a twisted state at a higher voltage, as described in GB 9607854.8, which will be referred to in this patent as a VCT device.
One problem with the twisted nematic device is that the contrast ratio of a normally white display remains at a low value until the voltage is increased to a value considerably higher than the threshold voltage. This is due to the nematic material close to the cell walls which does not fully reorient in the applied field due to the strong zenithal anchoring imposed by the surface alignment layer. This lack of surface reorientation also leads to higher voltage operation in the VCT device.
In respect of use of nematic and long pitch cholesteric materials for devices known as bistable nematic liquid crystal devices, the relevance of alignment and the problems associated therewith are as follows.
As described above, twisted nematic and phase change type of liquid crystal devices are switched to an ON state by application of a suitable voltage, and allowed to switch to an OFF state when the applied voltage falls below a lower voltage level, i.e. these devices are monostable. An advantage of nematic type of devices is that they have relatively low voltage requirements. They are also mechanically stable and have wide temperature operating ranges. This allows for the construction of small and portable battery powered displays. A disadvantage of such devices is that their monostable switching characteristic limits the number of lines that can be multiplex addressed.
Another way of addressing large displays is to use a bistable liquid crystal device. Ferroelectric liquid crystal displays can be made into bistable devices with the use of smectic liquid crystal materials and suitable cell wall surface alignment treatment. Such a device is a surface stabilised ferroelectric liquid crystal device (SSFELCDs) as described by:- L J Yu, H Lee, C S Bak and M M Labes, Phys Rev Lett 36, 7, 388 (1976); R B Meyer, Mol Cryst Liq Cryst. 40, 33 (1977); N A Clark and S T Lagerwall, Appl Phys Lett, 36, 11, 899 (1980). One disadvantage of ferroelectric devices is the relatively large voltage needed to switch the material. This high voltage makes small portable, battery powered displays expensive. Also these displays suffer from other problems such as lack of shock resistance, limited temperature range and also electrically induced defects such as needles.
If a bistable switching characteristic can be achieved using nematics then a display can be made which has the merits of both the above mentioned technologies but without their problems.
It has already been shown by Durand et al that a nematic can be switched between two alignment states via the use of chiral ions or flexoelectric coupling: A Charbi, R Barberi, G Durand and P Martinot-Largarde, Patent Application No WO 91/11747, (1991) xe2x80x9cBistable electrochirally controlled liquid crystal optical devicexe2x80x9d, G Durand, R Barberi, M Giocondo, P Martinot-Largarde, Patent Application No WO 92/00546 (1991) xe2x80x9cNematic liquid crystal display with surface bistability controlled by a flexoelectric effectxe2x80x9d.
U.S. Pat. No. 4,333,708 describes a multistable liquid crystal device in which cell walls are profiled to provide an array of singular points. Such substrate configurations provide multistable configurations of the director alignments because disclination must be moved to switch between stable configurations. Switching is achieved by application of electric fields
Patent Application No:. WO97/14990, (PCT-96/02463, GB95 21106.6) describes a bistable nematic device having a grating surface treatment to at least one cell wall that permits nematic liquid crystal molecules to adopt either of two pretilt angles in the same azimuthal plane. The cell can be electrically switched between these two states to allow information display which can persist after the removal of power.
Another bistable nematic device is described in GB.2,286,467-A. This uses accurately formed bigratings on at least one cell wall. The bigrating permits liquid crystal molecules to adopt two different angular aligned directions when suitable electrical signals are applied to cell electrodes e.g. dc coupling to flexoelectric polarisation as described in Patent Application No. WO.92/00546. Since in the two splayed states the director is quite close to being in the plane of the layer, the coupling between director and flexoelectric component can be small, which may hinder switching in some circumstances.
The bistable nematic device of GB2286467-A also has a further problem which is not present in ferroelectric devices, that is, the need to switch the surface layer of molecules in order to eliminate image sticking effects. Surface layer switching usually requires high voltages which leads to both high power consumption and the need for customised driver circuitry.
In respect of devices using smectic materials, the relevance of alignment and the problems associated therewith are as follows.
There are a number of devices based on smectic liquid crystal materials including:
A: Ferroelectric liquid crystals (usually SMC*).
One example of this is bistable, and is often termed a surface stabilised FLC device (SSFLC ref. N. A. Clark and S. T. Lagerwall, Appl. Phys. Lett., 36, 899 (1980). In this device planar aligned surfaces are arranged with parallel or anti parallel preferred alignment directions. The device is cooled from an overlying Smectic A phase into a bookshelf arrangement of the smectic layers, that is the material forms into micro layers arranged normal to the cell walls as in books on a shelf.
In the original teaching, the device used an unrubbed polymer surface alignment treatment to ensure the liquid crystal director n lies preferentially and substantially parallel to the surface plane (i.e. xe2x8axa5 to s, the surface normal). A preferred direction was then imparted by heating to the smectic A phase and shearing the layers in the required direction. The layers remained fixed on cooling into the SmC* phase. The surface energy is minimum for n xe2x8axa5 s so that two minimum energy states occur which can be selected by a suitable DC electric field.
Bradshaw and Raynes realised that improved SmA alignment for such a device resulted by having a chiral nematic N* phase above the SmA in which the pitch was sufficiently long for the surface forces to cause unwinding of the spontaneous helicity for a significant temperature range above the transition. They also required that the surface should be pre-treated to impart the preferred directions, often by use of parallel or antiparallel rubbing of a polyimide or polyamide layer; GB-2,210,469 U.S. Pat. No. 4,997,264, GB-2,209,610, U.S. Pat. No. 5,061,047, GB-2,210,468.
Later it was found that when a bookshelf aligned (where the layer normal is parallel to the plane of the dance i.e. xcex4=0) SmA sample is cooled into the SmC* phase, the layers become tilted in a chevron type of configuration; two type of chevron can exist and are defined as C1 and C2 type (ref J. Kanbe et al Ferroelectrics (1991) vol 114, pp3). These are shown in FIG. 18. This has been ascribed to the combined effect of shrinkage of the smectic layer spacing and pinning of the layers at the surface. The resulting chevron structure means that the director in the middle of the cell is (roughly) fixed in one of two orientations significantly less than the full cone angle. This means that with no applied field there is a substantial drop in the angle between the optic axis of the two xe2x80x9csurface stabilisedxe2x80x9d states which leads to a corresponding drop of the display brightness. A number of methods of improving the optical brightness have been proposed for practical devices:
1. AC field stabilisation:
An applied AC field pulse of insufficient time and voltage (xcfx84 V) to latch into the two states couples to the dielectric tensor (primarily the dielectric biaxiality) to increase the angle between these states and enhance the brightness. The main problem with this type of approach is that a high frequency voltage is constantly required to maintain the required brightness. This causes a high power dissipation, particularly for complex displays where the applied frequency is high. Usually the brightness is compromised by using a suitably low AC voltage. It has the advantage that, if C2U type alignment is used, there is no need for surface switching, and hence surface memory effects are minimal, and the slower switching at the surface does not affect the device.
2 High pre-tilt Parallel:
This geometry has (approximately) the same chevron structure with the director at the chevron interface also at a low angle to the rubbing direction. However, the director at the surface is at a much higher in-plane twist angle due to the competing effects of lying on the SmC* cone and with the preferred alignment pre-tilt. This type of device gives good brightness but suffers from a slower response since it involves surface switching, and from strong surface memory problems which may lead to image sticking.
3 Quasi-bookshelf:
Two methods may be used to reduce the layer tilt angle and thereby increase the device brightness. Pre-treating the device with a low frequency field of sufficient magnitude or choosing certain materials in which the layer shrinkage on cooling through the smectic phases is reduced (some materials may actually increase layer spacing on cooling). Such a device has similar advantages and disadvantages to the high pre-tilt configurations.
4 Uniform Tilted layer (High pre-tilt anti-parallel) geometry:
Similar to the previous two geometries, but there is no chevron (and therefore no constraint on the director at the cell centre) and the high angle between the bistable states is stabilised solely by the surfaces.
B: Electro-clinic optical shutters:
Application of a DC field to the smectic A (or other orthogonal smectic) phase of a chiral material leads to an induced tilt of the director and hence optical axis normal to the applied field. In a (approximately) planar aligned liquid crystal cell with electrodes on the substrate surfaces the electroclinic effect induces a rotation of the optic axis by an angle proportional to the applied field E. Thus, an optical shutter with full analogue amplitude or phase modulation may be obtained.
A common problem with such a device is obtaining suitably uniform and planar alignment of the smectic layers. A lesser problem is that the induced switching may involve some rotation of the director away from the preferred alignment direction at the surface. This movement is subject to a surface viscosity which may impede the switching time of the device and also to certain surface memory effects.
C: Anti-ferroelectric smectic liquid crystals (AFLC):
Certain materials form an anti-ferroelectric phase which may be used in active matrix or direct drive devices. Effectively these devices have a similar appearance to the smectic A phase until sufficient DC voltage is applied, above which the sample is in either of two states (depending on the polarity of the applied signal) similar to the normal ferroelectric phase.
There is a limited number of materials which form this phase (particularly over a wide temperature range) and all those found so far have direct isotropic to smectic phase (i.e. no overlying chiral nematic phase). This means that the materials are more difficult to align, forming batonnets (see Gray and Goodby book) of the smectic at this transition.
The mechanism for this is that the smectic layer structure nucleates in a limited number of xe2x80x9ccold spotsxe2x80x9d in the isotropic liquid. The layers then curve around this point to minimise the bend and splay of the layer normal. Where the layers meet the surface they become pinned and difficult to move. Hence, it is difficult to obtain the desired layer arrangement (e.g. planar or bookshelf) once the batonnet structure has pre-formed. On cooling into the AFLC phase, the applied field tends to induce twist of the director at the surfaces which also leads to problems associated with surface switching such as slower speed, surface memory effects, etc.
D: SmC* Optical Shutters:
Bradshaw and Raynes also described a type of device in which the FLC is obtained from cooling directly from the unwound No phase in a parallel rubbed device, preferably within applied DC field applied during the phase transition. The unwound N* phase has the director in the rubbing directions and on cooling into the SmC* this orientation is maintained and the layer normal twists through the angle xcex8. Degeneracy of the direction in which the layer normal is oriented is removed by the application of the DC field.
This is a monostable device, since it always relaxes back to the surface stabilised state (with n ∥ s) once the field is removed (it may be used in devices when the field is retained, either through AC stabilisation or through inclusion of TFTs or similar non-linear electrical elements at each pixel. However, it is fist (due to Ps). Primarily, switching occurs in the bulk of the cell and little or no switching occurs at the surface. However, this means that the director is highly twisted and non uniform in structure. This means that the optical appearance is poor (particularly if used in conjunction with a dye as done in early Hitachi work) and so this is a case where surface switching is required to improve performance. Also, alignment is difficult over a wide temperature range because layer shrinkage still occurs in many N-SMC* materials, leading to a chevron structure and associated defects.
Alignment of liquid crystals on a surface is therefore a significant problem for all these device types. Several different means are known by which liquid crystal fluids may be aligned on a surface. Evaporation of silicon monoxide from a direction at least 30xc2x0 from the plane of the substrate provides a surface which aligns a nematic liquid crystal in the plane of the substrate, along an axis orthogonal to the evaporation direction. In contrast, if the evaporation is conducted from a direction making an angle of about 5xc2x0 or less from the substrate, the resulting surface aligns a nematic liquid crystal along a direction tilted from the plane of the substrate by about 20xc2x0 in the direction of the evaporation source.
Many commercial liquid crystal devices are fabricated using rubbed polymer alignment layers, especially rubbed polyimide alignment layers. Typically such layers are deposited as an amide precursor polymer by spin deposition of a solution. After removal of the solvent, the polymer coating is imidised by baking at high temperature, then unidirectionally nibbed with a cloth. The resulting surface aligns liquid crystal materials along the direction of rubbing with a tilt out of the plane of the surface in the direction of rubbing. The magnitude of the tilt angle is typically 1xc2x0 to 2xc2x0, but special polyimide formulations and treatments are available which can provide higher magnitudes of pretilt. Some polymer layers are capable of aligning liquid crystal material when cross linked by exposure to linear polarised light (WO95/22075, GB-9444402516). This avoids the need for rubbing which is useful when substrates carry thin film transistors for a part of active matrix displays. The aligned polymer may also be used in conjunction with gratings as noted below.
A further means to provide a surface alignment for liquid crystal materials is available from the deposition of different surfactant materials onto the substrate from solution. A range of different surfactants may be used, including quaternary ammonium salts, alkylated silazenes and basic chromium alkanoates. Treatment of the surface usually entails dipping or spin coating with a dilute solution of the surfactant, and usually results in an alignment of the liquid crystal orthogonal to the plane of the substrate, termed homeotropic alignment. Binuclear chromium alkanoates and other binuclear surfactants may provide alignment in the plane of the substrate without any preferred direction in this plane.
Yet, a further method to achieve liquid crystal alignment at a surface involves fabrication of a relief structure such as a relief grating on the surface. Such a structure may be obtained by photolithographic means, by embossing a compliant surface layer such as a polymer against a master structure fabricated on, for example, a metal sheet, by mechanically scribing the surface or by other means. A grating structure aligns a nematic liquid crystal along the direction of the troughs and crests of the grating. More complex relief structures can provide tilted or bistable alignment.
The alignment methods of the known art suffer a number of shortcomings which prevent liquid crystal devices manufactured according to these methods from achieving their full potential utility.
One such shortcoming is that it is hardly possible according to known methods, to provide a surface alignment treatment on which the liquid crystal alignment is free to adopt any alignment direction in the plane of the surface. A planar alignment may be obtained by various methods including evaporation of an inorganic material from substantially normal incidence to the substrate, or by coating the substrate with a known polymer material such as a polyimide material without mechanical rubbing. In these cases, the alignment of the liquid crystal on the surface is not fixed during the surface preparation, but is fixed by the alignment of the liquid crystalline phase which first contacts it, and then becomes immovable.
On such a surface the alignment direction is determined by such factors as the flow direction or the direction of a temperature gradient or electric fields at the time the liquid crystal phase first contacts the surface. It is desirable to provide a surface treatment which can allow the liquid crystal alignment direction to rotate freely and repeatedly in the surface plane, but this is not available from known surface treatments.
A second shortcoming of known liquid crystal alignment techniques is that the energy required to change the zenithal angle between the substrate and the liquid crystal director is much greater than the elastic distortion energy of the liquid crystal itself which is generated by commonly applied voltages. This means that in liquid crystal devices using known alignment techniques, the liquid crystal director remains substantially fixed in tilt angle at the cell walls and the switching of the device which provides an optical effect occurring only in the parts of the device which are separated from the cell walls by some distance which depends on the magnitude of the applied field.
The present inventors have found that the above problems are reduced by a surface alignmlent treatment which allows movement of liquid crystal molecules at or close to the cell walls, hence the liquid crystal director is in contact with the wall to reversibly change its orientation at low values of applied field, for example at applied field strengths of the order of less than 1 volt per micron for an applied electric field. The benefits of such a surface treatment may include reduction in the operating voltage of the device and/or an improvement in the switching behaviour of the device such as the electro-optic threshold steepness of the device which determines the amount of information which may be written on an electro-optic display by means of the known methods of RMS multiplex driving.
Accordingly, in a first aspect the invention provides a liquid crystal device comprising a layer of a liquid crystal material contained between two spaced cell wall carrying electrodes structures and an alignment treatment on at least one wall, characterised by means for reducing anchoring energy at the surface alignment on one or both cell walls.
The anchoring energy reduced is one or more of azimuthal anchoring energy, zenithal anchoring energy, and translational anchoring energy (movement along the alignment treated surface). The significance of anchoring energies in the context of different device types are discussed further below. Further aspects of the invention relevant to specific device types are also discussed further below.
Anchoring energy arises from surface topography features such as grooves or gratings, and from chemical bonding interactions. The present invention reduces anchoring energy by changing the chemical bonding. Additionally the surface topography may also be changed, for example to reduce the dimensions of grooves or gratings. The means for reducing energy may be an oligomer or short chain polymer which is either spread on the surface or added to the liquid crystal material. The size of oligomer or short chain polymer may be selected to give a desired amount of preferential deposition at cell walls and slight separation from the liquid crystal material host.
The means for reducing anchoring energy may be an oligomer containing esters, thiols, and/or acrylate monomers and or which is either spread on the surface or added to the liquid crystal material.
The alignment treatment and means for reducing anchoring energy may be provided by a double layer treatment, now referred to as a substrate layer and a polymer layer. The substrate layer may either be formed in the surface of the cell wall, e.g. by mechanical rubbing of the surface, or (and preferably) be a coating on the cell wall. This coating may include anisotropic features which act to align liquid crystal phases placed in contact with it or in close proximity to it. Such features may include surface relief features including a plain or blazed grating or bigrating structure, or a regular or irregular array of surface features including but not limited to columns, tilted columns, platelets and crystallites e.g. formed by normal or oblique evaporation of inorganic materials onto the surface or by mechanical abrasion or working of the surface. Such features may also include a substantial anisotropy in the substrate formed, for example, by mechanical stretching or rubbing of the substrate layer or by exposure of the substrate layer to polarised actinic radiation.
The polymer layer (formed on the substrate layer) has the characteristics of having imperfect solubility in the liquid crystal material used in the device, of having a physical affinity for the surface of the substrate, and of retaining a substantially liquid like surface at the polymer/liquid crystal interface.
The polymer may be applied to the device in various ways. In one approach, the polymer is formed by polymerisation of reactive low molecular weight materials in solution in the liquid crystal fluid. The resulting solution or dispersion of polymer in liquid crystal is then filled into the cell, and the polymer is allowed to coat the substrate surfaces. Optionally, the dispersion of polymer in liquid crystal may undergo intermediate processes such as filtration or centrifuging prior to being filled into the display cell.
In a further approach to applying the polymer to the device the reactive low molecular mass materials may be dissolved into the liquid crystal which is then filled into the display cell. Polymerisation is then initiated by known means, such as by heating or exposure to short wavelength optical radiation in the presence of an initiator. After polymerisation the polymer is allowed to diffuse to and coat the substrate layers.
A still further approach to applying the polymer to the device is provided by polymerisation of the reactive materials in the presence or absence of an inert solvent. The solvent, if present, is removed and the resulting polymer is dissolved in the liquid crystal and filled in to the display cell.
A further approach to applying the polymer to the device is to form the polymer on the substrate by applying a thin layer of reactive low molecular weight materials to the substrate by known means such as by spinning a stoichiometric amount of each onto the substrate in solution in a solvent. After removal of the solvent, polymerisation is initiated by heating or by exposure to light in the presence of a polymerisation initiator. The treated substrates are then assembled into a cell and the liquid crystal added in.
The polymer is characterised in that it is substantially non-crystalline in the presence of the liquid crystal, and that it possesses a glass transition temperature below the operating temperature range of the device. The polymer may be substantially linear in its molecular structure or it may include branch points. The polymer may also be crosslinked to a low degree in order to promote phase separation from the liquid crystal and deposition onto the substrate, but such crosslinking is at such a level that a fluid, gum-like, gel-like or elastic character is retained, and the polymer does not present a hard glassy or solid like character which is retained on heating.
Preferred polymeric materials include thiol/ene polymers prepared by free radical polymerisation of known monomers in the presence of an added thiol compound which serves to limit the molecular weight of the product through chain transfer reactions. Details of suitable material are listed later.
In relation to twisted nematic devices, the present inventors have found that the contrast ratio of a twisted nematic device can be improved by using an additional a surface treatment which reduces the zenithal anchoring energy of the surface and thereby allows field-induced reorientation of the near-surface nematic layers. Such a treatment also has the added advantage of leading to a lowering of the threshold voltage. Lower voltage operation is preferable for both passive matrix and active matrix twisted nematic devices as it allows a display to operate with a lower power consumption.
Accordingly, in a second aspect the invention provides a twisted nematic liquid crystal device capable of being switched from a twisted state to a non twisted state comprising; two cell walls enclosing a layer of nematic liquid crystal material; electrode structures on both walls for applying an electric field across the liquid crystal layer; a surface alignment on both cell walls providing alignment direction to liquid crystal molecules and arranged so that a twisted nematic structure is formed across the liquid crystal layer at either zero volts or at a higher voltage; means for distinguishing between the two different optical states of the liquid crystal material; CHARACTERISED BY means for reducing zenithal anchoring energy in the surface alignment on one or both cell walls.
Additionally the azimuthal anchoring energy may also be reduced.
The means for reducing azimuthal anchoring energy and zenithal anchoring energy may be an oligomer containing esters, thiol, and/or acrylate monomers either spread on the surface or added to the liquid crystal material, e.g. the materials N65 and MXM035.
The oligomers may migrate preferentially to the surface in order to minimise the surface free energy. This may dilute the amount of liquid crystal at the surface leading to an effective reduction in the order parameter, S which is defined by (P. G. deGennes, The Physics of Liquid Crystals, Clarendon Press, Oxford 1974):   S  =            1      2        ⁢          ⟨              (                              3            ⁢                          cos              2                        ⁢            θ                    -          1                )            ⟩      
The order parameter is an indication of how well molecules align in a cell. Additionally the phase of the liquid crystal material at the surface may be changed by the oligomers, e.g., from nematic or long pitch cholesteric to isotropic.
The treatment may be used in conjunction with a surface which induces monostable pretilted nematic alignment.
The alignment layer may be a rubbed polymer surface as described in S. Ishihara et al., Liq. Cryst, vol.4, no. 6. p.669-675 (1989) or an obliquely evaporated inorganic material as described in W. Urbach, M. Boix, and E Guyon, Appl. Phys. Lett., vol. 25, no. 9, 479 (1974) or a polymer surface where in-plane anisotropy is achieved by illumination with polarised light such as M. Schadt et al., Jpn. J. Appl. Phys., v. 31, no.7, p.2155 (1992).
Alternatively, the alignment layer may be a surface-monograting with an asymmetric groove profile as described in G. P. Bryan-Brown and M. J. Towler, xe2x80x9cLiquid crystal device alignmentxe2x80x9d, GB 2,286,466A (GB9402492.4).
The alignment directions on the two surfaces may be substantially perpendicular.
The nematic liquid crystal may contain a small amount ( less than 5%) of a chiral dopant material e.g., R1011, CB15 Merck.
The cell walls may be substantially rigid e.g., glass material, or flexible e.g., polyolefin.
The electrodes may be formed as a series of row and column electrodes arranged and an x,y matrix of addressable elements or display pixels. Typically, the electrodes are 200 xcexcm wide spaced 20 xcexcm apart.
Alternatively, the electrodes may be arranged in other display formats e.g., r-xcex8 matrix or 7 or 8 bar displays.
In relation to bistable nematic devices, the present inventors have found that the problem of surface layer switching is reduced by using a surface treatment which changes the liquid crystal properties in the vicinity of the surface and so leads to a lower anchoring energy between the liquid crystal and the surface. This allows lower voltage operation without compromising other device parameters.
Accordingly, in a third aspect the invention provides a bistable nematic liquid crystal device which comprises; two cell walls enclosing a layer of nematic liquid crystal material; electrode structures on both walls; a surface alignment on both cell walls providing alignment direction to liquid crystal molecules; means for distinguishing between switched states of the liquid crystal material; CHARACTERISED BY means for reducing inelastic azimuthal memory anchoring energy in the surface alignment on one or both cell walls.
Ideally, the inelastic azimuthal memory anchoring energy is reduced to zero. Preferably, the zenithal anchoring energy is also reduced.
The means for reducing energy may be an oligomer or short chain polymer which is either spread on the surface or added to the liquid crystal material.
Preferably, the oligomer or short chain polymer does not change the pretilt by a substantial amount, e.g., change the pretilt by more than 5xc2x0.
The treatment is used in conjunction with a surface which induces bistable nematic alignment.
The bistable surface may be a surface alignment bigrating on at least one of the cell walls that permits the liquid crystal molecules to adopt two different azimuthal alignment directions, as in patent application WO097/14990, (PCT-96/02463, GB95 21106.6).
The angle between the alignment directions may be 90xc2x0 or less than 90xc2x0.
The grating may be a profiled layer of a photopolymer formed by a photolithographic process, e.g., M C Hutley, Diffraction Gratings (Academic Press, London 1982) p 95-125; and F Horn, Physics World, 33. (Mach 1993). Alternatively, the bigrating may be formed by embossing; M T Gale, J Kane and K Knop, J App. Photo Eng, 4, 2, 41 (1978), or ruling; E G Loewen and R S Wiley, Proc SPIE, 88 (1987), or by transfer from a carrier layer.
The bigrating may have a symmetric or asymmetric groove profile. In the latter case the surface induces both alignment and pretilt as described in GB2286467-A.
The gratings may be applied to both cell walls and may be the same or a different shape on each wall.
The bistable surface could alternatively be formed by using an obliquely evaporated material as described in patent Application WO 92/0054 (G Durand, R Barberi, M. Giocondo and P Martinot-Largarde, 1991).
The cell walls may be substantially rigid, e.g., glass material, or flexible e.g., polyolefin.
The electrodes may be formed as a series of row and column electrodes arranged and an x,y matrix of addressable elements or display pixels. Typically, the electrodes are 200 xcexcm wide spaced 20 xcexcm apart.
Alternatively, the electrodes may be arranged in other display formats, e.g., r-xcex8 matrix or 7 or 8 bar displays.
In relation to smectic devices, the inventors have found that problems in such devices may be reduced by use of a surfactant to lower the interaction between the surface(s) of cell wall(s) and the liquid crystal in the smectic phase, or in the overlying nematic phase from which the cell is cooled into the smectic phase for all operating temperatures. This use of a surfactant may be termed a slippery surface treatment. Thus, improved alignment, optical properties, switching speed, and stability to shock of smectic devices are achieved through slippery surface treatment.
Accordingly, in a fourth aspect, the invention provides a smectic liquid crystal device which comprises: a liquid crystal cell including a layer of smectic liquid crystal material contained between two walls bearing electrodes and surface treated to give both an alignment and a surface tilt to liquid crystal molecules; CHARACTERISED BY means for reducing anchoring energy at the surface alignment on one or both cell walls.
The means for reducing anchoring energy may be an oligomer containing esters, thiols, and/or acrylate monomers and or which is either spread on the surface or added to the liquid crystal material.
In its most elemental form the surfactant provides a slippery surface which reduces the interaction between the liquid crystal molecules and those of the surface of the cell wall (or alignment layer surface). Thus, the slippery surface may be thought of as having increased freedom for translational and rotational movement of the liquid crystal molecules closest to the surface. There are five surface terms (ref: Int Ferroelectric Liquid Crystal Conf(FLC95), Cambridge, UK, Jul. 23-27, 1995, vol.178 No.14 J. C. Jones, pp155-165) which are relevant and may be controlled by the surfactant:
(1) xcex1, zenithal anchoring energy. How easily the director surface tilt angle is changed (i.e., a rotational energy).
(2) xcex2, azimuthal anchoring energyxe2x80x94case of changing surface twist angle of director (i.e., a rotational energy).
(3) xcex3, related to the pretilt angle of the director at the surface.
(4) Layer pinning termxe2x80x94How easily layers may be moved across the surface (i.e. a translational energy). This is the macroscopic effect of the (partial) adsorption of liquid crystal molecules onto the surface layer reducing translational movement of the molecules and hence of the smectic layers.
(5) Polar surface energyxe2x80x94In ferroelectrics (or flexoelectrics) a term which is minimum for a particular orientation of the Ps at the surface.
In this aspect of the present invention each of these factors is influenced by the presence of a slippery surfactant which acts to separate the solid and liquid crystal regions by the induced changes of liquid crystal order close to the surface. For example, if nematic order exists close to the surface layer of a smectic device, then layer pinning is greatly reduced. If the cone angle is lower, surface switching is reduced, as well as the polar surface term.
Advantages provided by this aspect of the present invention are as follows:
(1) Reduced layer pinning hence control of the smectic layers is easier;
(2) Reduced nematic-like surface energies, hence orientation changes of the director at the surface are enhanced.
(3) Reduced adsorption of liquid crystal molecules at the surface, hence reduced surface memory effects and reduced surface viscosity;
(4) Reduced polarity of the surface, hence less coupling to the spontaneous polarisation coefficient (Ps) in ferro electric liquid crystal systems resulting in less T state formation.